1. Field of the Invention
This invention relates to a lens that permits a camera to be used for macro-photography by mounting it on the camera in the reverse direction.
2. Description of the Prior Art
Increasing the photographic magnification of a lens for macro-photography has involved shifting the lens to a close-up position nearer than the closest distance of the normal distance range of the lens.
For such macro-photography, the lens barrels of prior art lenses have been arranged, for example, as disclosed in the specification of U.S. Pat. No. 4,175,831. According to that patent, a movable lens is divided into a number of lenses and one of them is movable forward into a macro-photography zone.
In order to increase the magnification of a lens up to an actual size or a conformal magnification value, it has been necessary to extend the lens to a value equal to the focal length of the lens. This has now raised various problems including an increased outside diameter of the lens for preventing light flux from being eclipsed, and increases both in dimensions and weight resulting from lengthened multiplex structure of the lens barrel.
To solve these problems, one known arrangement limits the extension of the lens to one-half of its focal length to reduce the size of the lens barrel and to obtain magnifications up to a conformal magnification value. This is done by extending the lens to a distance corresponding to the other half focal length with an intermediate ring whose length is 1/2 of the focal length and is interposed in between the lens barrel and a camera body. An example of this arrangement is disclosed in U.S. Pat. No. 4,003,068. However, in such an arrangement the intermediate ring is totally unnecessary for photography within a range from infinity to a 0.5 magnification point or thereabout. Yet it is inconvenient to stow the immediate ring when the ring is not in use. Besides, the intermediate ring increases in the cost of the lens.